1. Field of the Invention
This invention relates generally to x-ray tubes of the rotating anode type and is concerned more particularly with an x-ray tube having a rotor structure with improved means for supporting a rotatable target disc.
2. Discussion of the Prior Art
A conventional x-ray tube of the rotating anode type includes a tubular envelope having transversely disposed therein an anode target disc with an outer annular portion called the focal track. The focal track is made of x-ray emissive material and has a radially sloped surface with a focal spot area disposed in spaced alignment with an electron emitting cathode. Electrons beamed from the cathode onto the aligned focal spot area penetrate into the underlying material of the focal track and generate x-rays which radiate from the focal spot area. Since most of the electron energy impinging on the focal spot area is converted into heat, the target disc is rotated thereby constantly changing the portion of the focal track in the focal spot area and allowing the heat to dissipate by radiation through the envelope of the x-ray tube.
Therefore, the target disc is supported for axial rotation by a bearing mounted rotor structure including an axially extending stem having one end portion attached to a central portion of the target disc. The stem usually is provided with a minimum cross-sectional size for rotatably supporting the target disc while restricting the flow of heat therefrom by conduction to the rotor structure. An opposing end portion of the stem is attached, generally by brazing, to a closed end of a tubular rotor skirt which is rotatably supported on a rotor shaft mounted in bearings.
However, it has been found difficult to produce between the closed end of the rotor skirt and the adjacent end portion of the stem a brazed joint which is sufficiently strong and durable for withstanding the stresses developed during rotation of the target disc. It may be found that, after an unexpectedly short time, the brazed joint will weaken and crack whereby the rotating target disc will commence to wobble and may damage the tube envelope. Also, the wobbling rotation of the target disc will adversely affect the bearings supporting the rotor shaft and may eventually cause permanent damage to the bearings.